1. Field of the Invention
The present invention relates to a microwave phase shifter which gives a desired phase shift amount to a high-frequency signal and a power amplifier using the microwave phase shifter.
2. Description of the Related Art
A microwave phase shifter is a circuit which gives a preset phase shift amount to a high-frequency signal of microwave, millimeter wave or the like and is normally configured by combining several transmission lines, a switch circuit and the like. For example, it has a transmission line used as a reference and transmission lines having delay amounts corresponding to preset phase differences with respect to the reference side transmission line, and a phase shift amount corresponding to the phase difference with respect to the reference is acquired by selecting one of the transmission lines by use of the switch circuit.
The microwave phase shifter with the above configuration is formed in an IC form by forming a plurality of transmission lines with different delay amounts and a switch circuit to switch the transmission lines on a substrate and thus an attempt is made to make the whole device small. However, since the switch circuit simultaneously makes selection of and switching to a single line from a plurality of lines on the input side and output side, a plurality of switch elements and driving control circuits are required. As a result, the circuit configuration of the microwave phase shifter formed on the substrate becomes complicated, the substrate becomes larger and the cost rises due to an increase in the number of manufacturing steps.
In the latest microwave communications devices for satellite communications, mobile communications, etc, a power amplifier using a semiconductor amplifier element is used, from the viewpoint of size, weight, reliability, etc. In a power amplifier using this semiconductor amplifier element, the output power which can be acquired by use of one element is not necessarily sufficient. Therefore, a power synthesizing type of power amplifier is proposed which, when a high output power is required, distributes an input signal into plural paths, amplifies them by use of semiconductor amplifier elements while controlling the signal phases, and then re-synthesizes the signals (for example, Jpn. Pat. Apln. KOKAI Publication No. 2001-196870 (p 5, FIG. 1)).
In the power amplifier, since a power loss occurs if the phases of the signals are deviated at the time of power synthesis, the phase differences between the signals are eliminated and the loss at the time of power synthesis is reduced by inserting phase shifters into paths other than a path used as a reference to adjust the phases. Thus, in the power synthesizing type of power amplifier, phase shifters corresponding in number to (the number of distributions—1) are required. Therefore, in order to make the power amplifier small and sufficiently reduce the loss, a phase shifter which is small and inexpensive and can relatively easily and precisely adjust the phase shift amount is desired.